1. Field of the Invention
This invention relates to generally to motor vehicle sensors, and more particularly an air fuel ratio sensor used for controlling an air fuel ratio of an automotive engine or the like.
2. Description of the Related Art
Conventionally, an air fuel ratio sensor is disposed in an exhaust system of an automotive engine, and combustion of the automotive engine is controlled based on the air fuel ratio detected by the sensor. Accordingly, purification efficiency of exhaust gas in a three-way catalytic converter or the like, which is also disposed in the exhaust system, can be improved.
The air fuel ratio sensor typically has a sensor element having a U-shaped cross-section and an inside chamber formed therein into which atmospheric air is introduced as reference gas. The sensor element is accommodated in a housing, and the housing is covered with a measurement gas side cover at a lower side thereof and with an atmosphere side cover at an upper side thereof. A heater is disposed in the inside chamber of the sensor element to rapidly raise a temperature of the sensor element up to an activation temperature. The heater is composed of a conventional insulating ceramic member holding therein a heating member made of W, W-Re, W-Mo, Pt, or the like which generates heat upon receiving electricity.
This kind of ceramic heater is, however, easily damaged by vibrations, impact and the like, resulting in deterioration of mechanical durability of the air fuel ratio sensor. Further, heat resistance of the heating member material, such as W or Pt, is not so large that the heater can be used at a temperature more than 1000.degree. C. for an extended period of time. Therefore, the temperature raising performance of the sensor element is low. As a result, so that the air fuel ratio sensor cannot exhibit a sufficient starting performance, meaning that the sensor is not capable of detecting the air fuel ratio immediately after the sensor element starts to be heated by the heater.
In addition, the temperature in the sensor element needs to be detected to ensure the durability, prevent damage, and improve sensor characteristics. However, the sensor element is so small that it is difficult for a temperature detector to be disposed in the inside chamber of the sensor element. This makes it difficult to detect the temperature in the sensor element when the air fuel ratio sensor is operated.